Data storage devices of the type known as "Winchester" or "hard" disc drives are typically provided with a plurality of rigid discs that are coated with a magnetizable medium and mounted on the hub of a spindle motor for rotation at a constant high speed. A controllably positionable actuator is disposed adjacent the discs, the actuator including a plurality of heads which are used during write and read operations to magnetically store and retrieve user data from tracks defined on the disc surfaces.
A closed loop servo system is used to control the position of the heads with respect to the tracks on the disc. More particularly, the actuator typically includes a coil of a voice coil motor (VCM) so that currents applied to the coil by the servo system cause the heads to move relative to the tracks in a controlled manner. A read/write channel, responsive to the heads, cooperates with an interface circuit to control the transfer of user data between the discs and a host computer in which the disc drive is mounted.
As will be recognized, consumer demand for ever increasing disc drive storage capacities and data transfer rates have led to continual advances in the disc drive art. One such advancement is the increasingly common use of magnetoresistive (MR) heads, each of which typically includes an MR element having a changed electrical resistance in the presence of a magnetic field of a particular orientation. Thus, during a disc drive read operation a bias current is passed through the MR element and the data stored on the corresponding track is detected as a function of changes in voltage across the MR element.
It will be recognized that increased storage capacities can be further achieved through the flying of the heads of a disc drive closer to the surfaces of the discs; however, a resulting problem is that the heads will occasionally contact the discs at high points on the disc (or the heads will contact contaminating particles disposed on the disc). Because of the sensitivity of an MR head, such contact will lead to a sudden increase in temperature of the head, distorting the readback signal for several microseconds. Such an anomalous condition is commonly referred to as a thermal asperity and will cause the readback signal to have a sudden increase in amplitude, followed by a longer falling edge due to the relatively large heat dissipation time constant of the MR head.
A typical read channel employs a variable gain amplifier (VGA) having an automatic gain control stage (commonly referred to as an "AGC") which continuously monitors and amplifies the readback signal to a level sufficient for processing by the rest of the read channel. Thus, a thermal asperity can cause the AGC to react by undesirably adjusting the gain of the VGA, introducing a gain error in the VGA of sufficient duration to prevent subsequent detection of timing information or user data by the read channel before the gain error can be corrected. This effect is particularly pronounced where the AGC has a relatively fast response to a signal amplitude above a target value and a slower response to a signal below the target value, so that the AGC responds to peak signal amplitude rather than average signal amplitude.
Accordingly, there is a continual need for improvements in the art whereby user data can be reliably recovered from a disc drive in the presence of anomalous conditions such as thermal asperities which tend to prevent the disc drive from properly detecting and retrieving the user data.